gdk: Add properties in GdkEventKey for the constructor
IBus GTK4 IM module needs to modify the state of GdkEventKey during the async filter_keypress(). Since GTK4 GdkEventKey is an object, the IM module retrieves all the member of GdkEventKey and can create a new GdkEventKey with the updated state in this way.
Fix #1208 (closed)